High strength steel alloy

ABSTRACT

An age-hardenable stainless iron-base alloy and method of making the same containing by weight about 9-13% chromium, about 8-16% cobalt, about 4-8% molybdenum, about 4-8% nickel in which those elements are balanced to provide an austenite retention index (ARI) value of from about 18.0 to 22.8 as calculated from the equation

United States Patent [191 Caton [451 Jan. 21, 1975 Related US.Application Data [63] Continuation-impart of Ser. No. 36,219, May 11,

1970, abandoned.

[52] US. Cl 75/128 B, 75/128 F, 75/128 N,

75/128 W [51] Int. Cl. C22c 39/20 [58] Field of Search 75/128 B, 128 W[56] References Cited UNITED STATES PATENTS 2,306,662 12/1942 Krivobok75/128W 2,432,618 12/1947 Franks 75/128 B 3,251,683 5/1966 Hammond75/128 B R26,225 6/1967 Kasak 75/128 B Primary ExaminerL. DewayneRutledge Assistant Examiner-Arthur J. Steiner Attorney, Agent, orFirmEdgar N. Jay

[57] ABSTRACT An age-hardenable stainless iron-base alloy and method ofmaking the same containing by weight about 9-13% chromium, about 816%cobalt, about 4-8% molybdenum, about 4-8% nickel in which those elementsare balanced to provide an austenite retention index (ARI) value of fromabout 18.0 to 22.8 as calculated from the equation ARl Ni 0.8(% Cr)0.6(% M0) 0.3(% Co).

8 Claims, 1 Drawing Figure HIGH STRENGTH STEEL ALLOY This application isa continuation-in-part of my copending earlier application filed May 11,1970, Ser. No. 36,219 and now abandoned.

This invention relates to an age-hardenable stainless steel alloy andmethod of making the same, and more particularly to a Cr-Co-Ni-Mostainless steel alloy having high strength, good ductility, and/or goodthermal stability.

Age-hardenable martensitic stainless steel alloys have hitherto beenprovided which combine highly desirable physical and chemicalproperties. The alloy of my U.S. Pat. No. 3,364,013 granted Jan. 16,1968 is one such alloy in which about 12-18% Cr, 1.5-3.5% Mo, and 18-26%Co are balanced in an iron matrix so as to provide a primarilymartensitic microstructure. No more than about 3% nickel is included inthat alloy in order to keep the amount of retained austenite below about20%. And in order to consistently attain high tensile strength at roomtemperature with maximum strength at elevated temperatures, the alloy isbalanced with no more than about 0.50% nickel present so as to containless than about 5% retained austenite.

Because nickel is such a powerful austenite former, it has been theusual practice as in said U.S. Pat. No. 3,364,013 as well as others,e.g., U.S. Pat. No. 3,154,412 granted Oct. 27, 1964, to exclude nickelor to restrict it to relatively small amounts even though when presentit imparts certain desirable properties to the alloy. For example, inU.S. Pat. No. 3,251,683 granted on May 17, 1966, it is brought out thatwhen cobalt is at about its lower limit of 3%, nickel can be present upto about 7% in order to enhance resistance to stress corrosion if theaccompanyng reduction in mechanical properties can be tolerated.

While all three of the aforementioned patents are concerned withmartensitic stainless steel, two of them rely upon balancing thesocalled nickel equivalent elements and the chromium equivalent elementsto establish the properties of the alloy while the remaining U.S. Pat.No. 3,154,412, though also containing the elements, Cr, Co, Ni, Mo andFe, relies upon substantial amounts of carbon and nitrogen to controlthe microstructure and thereby the properties of the alloy.

In U.S. Pat. No. 3,508,912, an age-hardenable steel is disclosedcontaining 2.5-6% Cr, up to 4% Ni, 8-15% cobalt, 10-18% Co Ni and withcarbon and nitrogen restricted to no more than 0.08%. The patent pointsout that larger amounts of chromium result in the formation ofobjectionable amounts of delta ferrite and also adversely affect theroom temperature and elevated temperature strength and ductility of thecomposition. Another age-hardenable steel is disclosed in U.S. Pat. No.3,650,845 containing 8-l4% Cr, 3-10% Ni, 4l0% Co, 2-5% Mo, and no morethan 0.06% C. This steel contains sufficient chromium to be consideredstainless and is balanced within the limits stated in accordance withtwo equations, one to limit the amount of delta ferrite present and theother to ensure a fully martensitic microstructure at room temperature,that is, an M, 100C.

It is therefore a principal object of this invention to provide animproved age-hardenable martensitic stainless steel alloy and a methodfor making the same which more consistently can be balanced to satisfythe requirements of modern-day technology. More specifically, it is anobject of this invention to provide such an alloy, and method of makingthe same, having high room temperature strength and toughness, and whichalso can be provided with elevated temperature strength, toughness andductility, that are not objectionably affected by prolonged exposure atelevated temperature.

Further objects as well as advantages of the present invention will beapparent from the following detailed description and the accompanyingdrawing which is a graph showing the interdependence of the alloys roomtemperature ultimate tensile strength with variations in its compositionas measured by its austenite retention index (ARI) in accordance withthe present invention.

The present invention also includes a new method for making thisessentially martensitic stainless steel as well as a new alloy producedthereby consisting essentially of, in weight percent,

All or part of the molybdenum can be replaced by an equivalent amount oftungsten.

and the balance essentially iron in which the elements nickel, chromium,molybdenum and cobalt are added in such proportions that the austeniteretention index ranges from about 18.0 to about 22.8 as calculated fromARI Ni 0.8(% Cr) 0.6(% Mo) 0.3(% Co) and with the elements carbon,nitrogen, manganese and silicon controlled as pointed out hereinafter.The relationship among the elements Ni, Cr, Mo, and Co which isidentified herein as the austenite retention index is based upon therelative effect of those elements in depressing the M, temperature ofthe alloy with the effect of nickel equal to unity. For the stated broadrange of this alloy, chromium was determined to be as effective indepressing the temperature (M at which the transformation of austeniteto martensite begins, molybdenum was found to be 60% as effective asnickel, and cobalt was found to be 30% as effective as nickel.

By carefully balancing the alloy in this way, an agehardenable stainlesssteel alloy is provided which, depending upon the amounts of chromiumand nickel selected, is characterized by one or more of the followingproperties, (1) a room temperature ultimate tensile strength of about225,000 psi to 300,000 psi, (2) a ratio of notch tensile strength toultimate tensile strength of at least 1, and (3) thermal stability suchthat mechanical properties undergo a minimum of change on exposure totemperatures of up to about 900F for long periods (e.g., 1,000 hours).

The elements nickel, chromium, molybdenum, cobalt and iron work togetherbut unless maintained within the stated ranges and carefully controlledto provide an austenite retention index within the values specified, thedesired properties cannot be consistently attained. The elements nickeland cobalt are austenite formers, nickel being much stronger thancobalt. Both chromium and molybdenum are ferrite formers but stabilizeaustenite. These factors are taken into account in the manner in whichthe austenite retention index is computed to provide an essentiallymartensitic microstructure in the age-hardened condition which issubstantially free of retained delta ferrite and which contains no morethan a tolerable amount of austenite. Preferably there is about toaustenite present to provide maximum toughness with little or no adverseeffect on tensile strength. That austenite is normally made up ofretained austenite which occurs as a result of solution treatment andreverted austenite which is formed furing the aging treatment.

Of the main alloying elements, nickel is the strongest austenite formerand, as was noted, is assigned a value of unity in my austeniteretention index. Because it has such a strong effect in depressing theM, temperature, nickel is limited to no more than about 8% while aminimum of 4% nickel is required to ensure the desired essentiallymartensitic microstructure by offsetting the effect of theferriteforming elements such as chromium and molybdenum. Within itsbroad range, the nickel content is adjusted depending upon the intendeduse for the parts to be fabricated from the alloy and the amount of theother alloying elements, particularly chromium and molybdenum, that ispresent. Preferably nickel is present in an amount ranging from about 5%to 7%. For best room temperature corrosion and/or tensile properties inthe case of parts that will not be subjected to prolonged exposure atelevated temperatures in service, this alloy is balanced with thesmaller amounts of nickel and the larger amount of chromium within thestated ranges while maintaining the austenite retention index withinspecified limits. On the other hand, when maximum tensile properties arerequired in parts intended for prolonged exposure to elevatedtemperatures of up to about 900F, then the larger amounts of nickel arebalanced with the smaller amounts of chronium within the stated ranges.

Chromium imparts stainless properties to the alloy. While as little as9% chromium can be used, preferably about 9.5-11.5% is used, while nomore than about 13% chromium is present, to ensure good stainlessproperties and enhance response to age hardening.

Cobalt, like nickel, is an austenite-forming element, but in this alloyhas less than about one-third the effect in depressing the M temperatureas an equal weight percent of nickel. Thus, larger amounts of cobalt canbe used without overbalancing the alloy to work with the other elementsto provide high strength and notch tensile strength. Cobalt may rangefrom about 8 to 16% in balancing my alloy over its broad range toprovide the desired austenite retention index. Preferably about 9.5l3.5%cobalt is present.

Molybdenum in an amount ranging from about 4% to 8% is essential to thestrength of my alloy. Apparently, the larger amounts of molybdenum workwith the larger amounts of nickel to provide strength properties whichare not adversely affected by prolonged exposure to elevatedtemperatures of up to about 900F. For best results in providing the bestcombination of strength, toughness and stability at elevatedtemperature, about 5% to 6% molybdenum is used.

It is to be noted that tungsten can be substituted for molybdenum inthis alloy. At any level of molybdenum, the amount of tungsten requiredto replace a given amount of molybdenum with an equivalent effect is inthe proportion of about 1.2% to 1.6% tungsten to 1% molybdenum.Therefore, throughout this application it is to be understood that whenmolybdenum is referred to, it is intended to include molybdenum andtungsten either together or individually with the tungsten replacing allor part of the molybdenum in the proportion stated.

Carbon and nitrogen are not considered to be desired additions in thisalloy, and each is preferably held to no more than about 0.01% becauseeach is a very strong austenite former when present in solid solutionabout 30 times more effective in that regard than an equal amount ofnickel. When carbon or nitrogen is present in solid solution in anamount of about 0.01% or less, the effect of either or both isnegligible and can be ignored. In the age-hardened condition of thealloy, a substantial part of the carbon and nitrogen is not present insolution but is present in the form of precipitated carbides andnitrides and so do not affect the presence of austenite. In amounts inexcess of about 0.1%, carbon may have a beneficial effect upon theultimate tensile strength, but it excessively impairs the yield strengthand results in the presence of relatively hard martensite in theas-solution-treated condition of the aloy which detracts from itsformability and machinability. When the most favorable notch tensilestrength is not required, carbon may be present in an amount of up toabout 0.1%, but then the amount of carbon in solution multiplied by 30must be added in calculating the ARI value when balancing the alloy toavoid the presence of excessive austenite in the as-age-hardened condition. Like carbon, nitrogen may also be present in amounts up to about0.1%; but when present in amounts greater than 0.01%, the amount insolid solution multiplied by 30 should be added in computing theaustenite retention index.

Manganese may be used as a deoxidizer if the alloy is melted in air, butis not considered a desirable alloy ing addition in this alloy becauseit is a relatively strong austenite former, about one-half as effectiveas an equal weight percent of nickel. Insofar as the calculation of theaustenite retention index of this alloy is concerned, manganese inamounts of about 0.5% or less can be ignored although manganese ispreferably kept below about 0.1%. Up to about 2% manganese can betolerated in the alloy, but above 0.5% the manganese present should betaken into account when calculating the austenite retention index byadding one half of all the percent manganese.

Silicon can also be used as a deoxidizer when the alloy is melted inair. Silicon is a strong ferrite former, being 1.5 times as effective asan equal weight percent of chromium. Though when present silicon affordssome oxidation resistance, it is not considered a desirable alloyingaddition but can be tolerated up to about 1%. Above about 0.5% siliconmust be taken into account when calculating the austenite retentionindex and is adjusted for by adding an amount equal to 1.5 times theamount of silicon present. In amounts less than about 0.5%, silicon canbe ignored when calculating the austenite retention index and preferablysilicon is kept below about 0.1%.

A relatively small amount of boron is beneficial in this alloy when besttoughness properties are desired. In that event, the alloy preferablycontains about 0.001% to 0.003% boron. When optimum toughness is resultin an effective amount of carbon in solution cither the aged or theaged-and-exposed condition inso far as the austenite retention index isconcerned (by exposed is meant heated at about 900F for 1.000

not q then P 10 8 ut 0.02% boron can be inhours), and no adjustment inthe ARI value was found cludedm the alloy because of its beneficialeffect upon to be necessary because f h carbon Content the ult1matetensile strength, the 0.2% y1eld strength and the Stress ruptureStrength of the alloy. Three notch and three smooth tenslle specimensThe alloy is readily prepared and formed into parts. were machmed d.tested at room iempcmture for It can be melted in air in the usual way,but better re- 10 g of q (1) 3 se i sults are attained when the alloy isvacuum-induction age an E gg gifi ggg melted. Preferably, adouble-melting process is used in temperature 0 out or f tut which aningot is air or vacuum-induction melted and Speclmen followlpg Solunor,treatment wah rough cast, and then remelted under vacuum or a controlledmachmed 0010 'f overslze then after havmg atmosphere as a consumableelectrode. Whether the aged or after havmg been aged and exposedto 900 Falloy is air or vacuum melted, only relatively simple F 1000 W as theCase may h was i" heat treatment is required to bring out the uniquepropmachmed' The Smooth bar specmens f p erties of the alloy. The alloycan be solution treated Carrymg 9" room temperature meflsuremems o unfrom about 1,4000 to ZOOOOF! preferably from about matetensile-strength (UTS), 0.2% yleld strength (0.2% 1,500 to 1,800F, for asufficient time to ensure compefcent elonganon Song) percent pleteaustenitizing. Usually about 1 hour for each inch Fiucnon m area RA) hada gauge dlameier of 0252 of thickness is sufficient. After cooling toroom temperand a gauge length of Notch, tensfle Strength atul'e, it isreheated 16 about 900 to 1,100F, prefera- ,(NTS) specmlens had a gauge0357 bly 6 to 1 OOOOF for about 2 to 4 hours followed by in., a notchdiameter of 0.252 in. and a notch root racooling in Cooling below roomtemperature during dius of 0.001 in., a stress concentration factor (K,)of any stage of the heat treatment is not required. In the about case ofwork pieces of substantial thickness, rapid cool- Th results f il testsi d out at room g from the auslenitiling e p as y q nchperature onspecimens of Examples l-6 in the solution ing in oil or water, is usedto ensure maximum response treated and aged condition are set out inTable II. The to the age-hardening treatment. results under 0.2% yieldstrength are in each instance The following illustrative examples ofthis alloy, the an average of two tests, and the remaining values areanalyses of which in weight percent are set forth in the average ofthree. In Table III are set out the results Table I as well as thefurther examples given hereinafof tests carried out at room temperatureon specimens ter for purposes of comparison, (unless otherwise indiofExamples 1-6 which, following solution treatment cated) were prepared asl7-pound experimental heats and aging, were then exposed to atemperature of using vacuum-induction melting, were cast into ingots900F for 1,000 hours. Again, the 0.2% YS data are the 2 3/8 in. sq. andthen forged using a furnace temperaaverages of two tests each while theremaining results ture of about 2,100F into 5/8 in. sq. bars which werewere obtained by averaging the measurements of three then formed intothe required test specimens. Heat tests in each case. Hardness data andthe percent austreatment was carried out at about 1,700F for 1 hourtenite present as determined by means of standard followed b uenchin inwater, then heatin at about X-ra diffraction techni ues for each ofthree condi q 8 g y 1 1,000F for 4 hours followed by an cooling. tions((1) as solution treated, (2) as solution treated TABLE 1 Example No. 12 3 4 5 0 ARI 19.3 21.3 21.0 21.2 22.5 22.0

In examples 1-6, the balance was iron except for inciand aged, and (3)as solution treated, aged and exdental impurities. In preparing each ofthe composiposed) are set out in Table IV.

tions, the amounts of each of the elements Cr, Ni, Mo, and Co added wereadjusted so as to provide an austenite retention index for each lessthan about 22.8 but not less than 18.0 according to the relationship.

ARI Ni 0.8(% Cr) 0.6(% M0) 0.3(% Co) TABLE II Ex. .2% Y8 UTS NTS No.X1000 X1000 X 1000 Elong. RA

psi psi psi TABLE lI-Continued Ex. .2% Y UTS NTS No. X1000 X1000 X1000Elong. RA

psi psi psi Result of one test.

' Average of two tests.

TABLE III Ex. .2% Y5 UTS NTS No. X1000 X1000 X1000 Elong. RA

psi psi psi TABLE IV Hardness R Austenite Ex. Solution Solution No.Treated Aged Exposed Treated Aged Exposed 1 22,5 46.5 48,5 N.D." N.D.N.D. 2 27.5 48 49.5 4 12 19 3 28.5 48 51 N.D. 4 5 4 31.5 51.5 53 N.D. 412 5 29.5 48.5 49 ll 21 27 6 31.5 55.5 55 5 12 Average of two testsexcept the exposed hardnesses of Exs. 3, 4 and 6. None detected Example1 illustrates the good tensile properties that are readily obtained withthe alloy of this invention. The hardness of about Rockwell C 23 in theassolution-treated condition provides good formability while the simpleaging treatment brings out a hardness of about Rockwell C 46.5.

The less than 10% increase in ultimate tensile strength and about 10%reduction in notch tensile strength between the results shown in Tables11 and Ill reflect good thermal stability in spite of the relatively lownickel content of 5.26%. The ultimate tensile strength of 240,000 psi isapproximately the average value to be expected from the calculatedaustenite retention index of 19.3 as can be observed from the drawing.Example 2 differs from Example 1 primarily in the increased nickelcontent although the increase in molybdenum content to 5.48% would beexpected to have a noticeable effect in increasing the stength of thealloy as compared to Example 1. However, it will be observed that theroom temperature ultimate tensile strength of Example 2 in the as-agedcondition is essentially the same as that of Example 1, and this isbelieved to be attributable to the fact that the increase in molybdenumwhich tends to increase the strength is offset by the increase in nickelcontent which, by increasing the austenite content, reduces the ultimatetensile strength. A comparison of the tensile data in Tables 11 and 111shows that Example 2 is more stable after the prolonged expposure atelevated temperature than Example l. The relatively small change intensile properties is indicative of the outstanding thermal stability ofthis composition. On the other hand, the effect of increasing thechromium content of Example 1 by about 2% as in Example 3, withoutincreasing the nickel content, makes the alloy thermally unstable as isclearly demonstrated. The alloy of Example 3 is well suited for use atroom temperature where high yield strength, ultimate tensile strength,notch tensile strength and good corrosion resistance are required, butis not intended for use where prolonged exposure to elevated temperaturewould be encountered. Example 4 with an ARI value of 21.2, very close tothe value of 21.3 for Example 2, demonstrates an outstanding degree oftensile strength and thermal stability. The increased strength ofExample 4, both the 0.2% yield strength and ultimate tensile strength,is believed to be attributable to the molybdenum content of about 7% ascompared to the approximate 5.5% molybdenum content of Example 2.Example 5 illustrates a composition prepared in accordance with thepresent invention so as to have an austenite retention index of 22.5 ata somewhat higher carbon level than preferred which has good tensilestrength and good thermal stability. Example 6 with an austeniteretention index of 22.0 is essentially the same composition as Example 5except that instead of about 7% nickel and 5% molybdenum, Example 6contains about 5% nickel and 7% molybdenum. The outstanding yield andultimate strength of Example 6 is believed to be primarily the result ofthe increased molybdenum content, the cobalt level of about 15% beingused to establish the desired ARI value. Thus, at a given value of theaustenite retention index, one reason for the vertical dispersion (inultimate tensile strength values) resides in the relative proportions ofnickel and molybdenum present in the composition. The composition ofExample 6 is well suited for use where its exceptional ultimate tensilestrength and 0.2% yield strength are required. However, othercompositions, Example 1 or Example 2, would be preferable where highernotch tensile strength is desired and in particular where a NTS/UTSratio of at least equal to one is desired.

The relationship between the austenite retention index as calculatedfrom AR1= Ni 0.8(% Cr) 0.6(% Mo) 0.3(% Co) and room temperature ultimatetensile strength of the alloy in its as-aged condition will be betterappreciated by reference to the drawing where the calculated values ofthe austenite retention index have been plotted along the abscissa andstress in thousands of pounds per square inch is plotted along theordinate. Curves A and B are respectively the envelopes of the maximumand minimum values of the results of room temperature ultimate tensilestrength tests carried out on specimens of about 50 differentcompositions. The various heats and the specimens thereof were preparedand the tests were carried out as described in connection with Examplesl-6. Curve C in dashed line is a curve of the average values at eachlevel of the austenite retention index. Curves A, B and C clearlyreflect the close interdependence of room temperature ultimate tensilestrength with the austenite retention index as calculated in accordancewith the present invention.

The steepness and relatively close spacing of the curves A, B and C foraustenite retention index values between 22.5 and 23.2 demonstrates thecare with which the alloying elements must be balanced. This may bebetter appreciated from the three following heats, identified as Alloys7, 8 and 9, when compared with Examples 16. Alloys 7-9 and testspecimens thereof were prepared as described in connection with Examplesl-6. Alloys 7, 8 and 9 had the following composition in weight percent:

TABLE VI Alloy .2% Y UTS NTS No. X 1000 X1000 X1000 Elong. RA

psi psi psi The adverse effect upon the 0.2% yield strength and ultimatetensile strength of a composition balanced so that the austeniteretention index is above 22.8 is clearly demonstrated by Alloys 7, 8 and9. Alloy 8 is essentially the same composition as Example 5 except forthe increase in the molybdenum content of about 1 percent and aresultant increase in the austenite retention index from 22.5 to 23.0.The loss in strength of Alloy 9 as compared to Example 5 is all the moresignificant when it is borne in mind that molybdenum would have had anoticeable strengthening effect but for the fact that the increaseserved to overbalance the alloy. The effect of the individual elementsin causing a nonhomogeneous microstructure, or what may be termed apatchy effect (because of austenite retention and/or reversion), isevident from a comparison of Alloys 8 and 9 which have a significantdifference in 0.2% yield strength and ultimate tensile strength.

In Table VII are set forth the results of room temper ature tensiletests carried out on specimens of each of the Alloys 7, 8 and 9 whichhad been solution treated, aged and exposed, all as was described inconnection with Examples 1-6 (see Table III).

In Table VI there is set forth hardness data and the percent austenitepresent as determined by means of standard X-ray diffraction techniquesfrom specimens of Alloys 7, 8 and 9 for each of the three conditions ashad been previously set out in Table IV in connection with Examples l-6.

TABLE VIII Hardness Austenite Allo solution Ex- Solution Ex- No. TreatedAged posed Treated Aged posed 7 97.5 R, 35.5 R 28 R, 13 67 84 8 91 R,95.5 R,, 97.5 R 16 71 60 9 24.5 R, 42 R 39.5 R, 24 42 45 When comparingthe data in Tables V-VIII with that set forth in Tables I-IV, it isimportant to keep in mind that each of the Alloys 7, 8 and 9 fallswithin the composition range of the alloy of the present invention. Itis only by carefully following and using the austenite retention indexin balancing the composition within the specified ranges for the variouselements that the outstanding results characteristic of the presentinvention can be attained.

The alloy of this invention is suitable for a wide variety of demandinguses. For example, it is especially well suited for making such parts ascompressor discs in jet engines because of its outstanding thermalstability combined with ultra high strength and ductility both at roomtemperature and at elevated temperatures. For such uses, the alloy, inaddition to being balanced by using the austenite retention index so asto obtain optimum strength, also has the lower amounts of chromiumcombined with the larger amounts of nickel. This alloy is alsooutstanding for use in forming fasteners and structural members, such asare used in aerospace vehicles, which are not subjected in use totemperatures higher than about 500F. For such uses, where thermalstability is not required, to provide the best combination of strengthand corrosion resistance, the higher amounts of chromium are balancedwith the lower amounts of nickel.

The terms and expressions which have been employed are used as terms ofdescription and not of limitation, and there is no intention in the useof such terms and expressions of excluding any equivalents of thefeatures shown and described or portions thereof, but it is recognizedthat various modifications are possible within the scope of theinvention claimed.

What is claimed is:

1. An age-hardenable stainless iron-base alloy which in its age-hardenedcondition is characterized by an ultimate tensile strength of about225,000 psi or more with good ductility and which consists essentiallyin weight percent of about Carbon 00.l Nitrogen 0-0. 1 Manganese 0-2Silicon 0-1 Phosphorus 00.05 Sulfur 00.05 Boron 0-0.02 Chromium 9-13(obalt 8-16 Molybdenum 4-8 Nickel 4-8 up to about 12.8% tungsten as areplacement for all or part of the molybdenum content in the ratio ofabout 1.2% to 1.6% tungsten to 1% molybdenum, the balance beingessentially iron and incidental impurities, the elements nickel,chromium, molybdenum and cobalt being balanced to provide an austeniteretention index (ARI) value of from about 18.0 to 22.8 as calculatedfrom the equation ARI Ni 0.8(% Cr) 0.6(% Mo) 0.3(% Co) by 30 is added.

2. The alloy is set forth in claim 1 which contains less than about0.01% carbon.

3. The alloy as set forth in claim 2 which contains less than about0.01% nitrogen.

4. The alloy as set forth in claim 1 which contains less than about0.01% carbon, less than about 0.01% nitrogen, less than about 0.1%manganese and less than about 0.1% silicon.

5. The alloy as set forth in claim 1 which contains about 9.511.5%chromium, about 9.513.5% cobalt, about 56% molybdenum, and about 57%nickel.

6. The alloy as set forth in claim 5 which contains less than about0.01% carbon, less than about 0.01% nitrogen, less than about 0.1%manganese, and less than about 0.1% silicon.

7. The alloy as set forth in claim 6 which contains about 0.0010.003%boron.

8. The alloy as set forth in claim 7 which contains less than about0.01% sulfur and less than about 0.01% phosphorus.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO.3,861,909

DATED I January 21, 1975 INVENTOR(S) 1 Robert L. Caton it is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Col. 1, line 35, for "accompanyng" read accompanying Col. 3, line 16,for "furing" read during line 25, for "ferriteforming" readferriteforming line 43, for "chronium" read chromium Col. 4, line 30,for "aloy" read alloy Col. 6, line 1, after "solution", insert in line17, for "finished" read finish Col. 7, line 64, for "expposure" readexposure Col. 11, line 19, after "carbon", for "is" read in Col. 12,line 1, for "is" read as Signed and sealed this 6th day of May 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officerand Trademarks

2. The alloy is set forth in claim 1 which contains less than about0.01% carbon.
 3. The alloy as set forth in claim 2 which contains lessthan about 0.01% nitrogen.
 4. The alloy as set forth in claim 1 whichcontaiNs less than about 0.01% carbon, less than about 0.01% nitrogen,less than about 0.1% manganese and less than about 0.1% silicon.
 5. Thealloy as set forth in claim 1 which contains about 9.5-11.5% chromium,about 9.5-13.5% cobalt, about 5-6% molybdenum, and about 5-7% nickel. 6.The alloy as set forth in claim 5 which contains less than about 0.01%carbon, less than about 0.01% nitrogen, less than about 0.1% manganese,and less than about 0.1% silicon.
 7. The alloy as set forth in claim 6which contains about 0.001-0.003% boron.
 8. The alloy as set forth inclaim 7 which contains less than about 0.01% sulfur and less than about0.01% phosphorus.